KrF excimer lasers are rapidly becoming the most important light source for integrated circuit lithography. Although these lasers are very complicated machines, their reliability has greatly improved during the past few years, and they are currently being integrated into full-scale integrated circuit production.
A detailed description of a KrF laser system is described in U.S. Pat. No. 4,959,840 issued Sep. 25, 1990 (incorporated herein by reference), and assigned to Applicants' employer. As explained in that patent, the excimer laser gain medium is produced by electric discharges between two elongated electrodes in a flowing gas medium which may be a combination of krypton, fluorine and a buffer gas, neon. The proportions are typically 0.1 percent fluorine, 1.0 percent krypton and the rest neon. The operating pressure is about 3 atmospheres. FIG. 1 is a drawing showing the important features of such a laser.
Typical lithography lasers currently being sold today for lithography operate at a high pulse rate of about 600 to 1,000 Hz. This is the reason it is necessary to circulate the laser gas through the space between the electrodes. This is done with tangential blower located below the electrodes in the laser discharge chamber. The laser gases are cooled with a heat exchanger also located in the chamber. Commercial excimer laser systems are typically comprised of several modules which may be replaced quickly without disturbing the rest of the system. Principal modules are shown in FIG. 1 and include:
Laser Chamber 8, PA1 Pulse Chamber 8, PA1 Pulse Power Module 2, PA1 Output coupler 16, PA1 Line Narrowing Module 18 PA1 Wavemeter 20 PA1 Computer Control Unit 22
The energy per pulse from these lasers in about 10 mJ and the duration of the laser pulses is about 15 ns. Thus, the average power of the laser beam at 600 to 1,000 Hz is about 6 to 10 Watts and the average power of the pulses is in the range of about 700 KW. FIG. 2 is a drawing showing the principal elements of the high voltage power supply for the laser. The voltage on capacitor Co is delivered by a 1 kv power supply in the form of DC pulses in the range of about 500 to 800 volts and is called the "charging voltage". The voltage on capacitor Cp which is also the voltage across the electrodes is also a DC pulse in range of about 12,0000 to 20,000 volts and is called the discharge voltage. The high voltage power supply system compresses the charging voltage pulse from a duration of about 0.7 ms to produce a discharge voltage pulse of about 200 ns. The discharge voltage is approximately proportioned to the charging voltage.
It is known that within the normal operating range of the KrF laser, output pulse energy can be increased by increasing the pulse discharge voltage; and it can be increased by increasing the fluorine concentration. Increases or decreases in both or either of these parameters is easily accomplished with these narrow band KrF excimer lasers; however, the pulse discharge voltage can be changed very quickly and accurately (i.e., a time intervals of less than 1 millisecond with an accuracy of less than 1%.) whereas changing the fluorine concentration requires much more time and the fluorine concentration controls are not currently very accurate.
Fluorine gas is extremely reactive, and in spite of great efforts to utilize materials which are compatible with fluorine, reactions occur continuously in the chamber depleting the fluorine, especially during and immediately following the electrical discharges during which time the fluorine is ionized.
A typical operating plan for producing constant laser pulses is to compensate for the fluorine depletion by increases in the discharge voltage. This is accomplished with a feedback control which monitors pulse energy on a "per pulse" basis at pulse frequencies such as 1,000 Hz and controls the voltage to maintain substantially constant pulse energy as the fluorine concentration decreases over time and increases when new fluorine is injected. Normally the operating plan will encompass a voltage control range so that when the charging voltage increases needed to compensate for the depleted fluorine, reach an "upper limit" (usually requiring a period of about two hours), a quantity of fluorine is injected during a period of a few seconds. The quantity injected is predetermined to correspond roughly to the quantity which would have been depleted over the two-hour period. During the fluorine injection period, the automatic feed back control will force the voltage down in order to keep pulse energy substantially constant so that at the end of the injection period the voltage is approximately at the low level of the voltage operating range and fluorine concentration is approximately at its high level. Prior art techniques are in use in which this injection is performed automatically by the control system for the laser. The start of the injection process is triggered by the charging voltage reaching or exceeding the above upper limit. During the next two hours, the process will repeat, and this general process may continue for several days. FIG. 3 shows a graph of average voltage as a function of pulse count for an operating unit. Note that at a pulse rate of 1,000 Hz, 1 million pulses correspond to about 16 minutes, and that for continuous operation, the injection period would be at intervals of about 1.3 hours, corresponding to about 5,000,000 pulses. (Often these lasers run at a duty factor of about 20-60 percent. This is because the lasers normally do not operate when the lithography tool is changing positions, which increases the time interval between injections to about several hours.)
Typical KrF lasers have a fairly broad possible range of operation in terms of discharge voltage and fluorine concentration within which a desired pulse energy can be achieved. For example, in one such laser the charging voltage range is from 567 volts to 790 volts, with the corresponding fluorine pressure range being 36.5 kPa to 18.5 kPa. A lithographer may choose any operating range within the laser's charging voltage range of for example, 567 volts to 790 volts. Prior art procedures for selecting the range have not been well thought out. One manufacturer has recommended operation at 75% of the maximum voltage. Another suggested the range be determined based on pulse energy transfer efficiency. Choosing a range that is solely determined by a maximum energy transfer efficiency or within some specific voltage range may severely limit the operating range of the laser. Also, as described in U.S. Pat. No. 5,142,166; by using energy recovery circuits, even if the laser is not operated in maximum energy transfer efficiency regime, the residue energy in the circuit can be recovered for the subsequent pulse. Choosing the correct range can be important because both the operating life of the laser is adversely affected by increased fluorine concentration and also by increased discharge voltage.
Pulse energy variation, called "energy sigma" is very important to integrated circuit lithographers. Energy sigma is the standard deviation from the mean energy of pulse energy in a series of pulses. Specifications on energy sigma are typically in the range of about 3 percent, but desired values may be much smaller. Prior art excimer lasers comprise feedback voltage control which automatically adjusts the discharge voltage of each pulse based on the monitored energy of the preceding pulse or pulses. Lithography lasers may be operated in a continuous mode at pulse rates such as 1,000 Hz in which case it is relatively easy to keep energy sigma small utilizing the pulse energy-voltage feedback control. However, a more typical mode of operation is called the burst mode in which the laser is operated at a pulse rate of 1,000 Hz for about 110 pulses followed by a dead time of a fraction of a second such as about 0.2 second, to a few seconds or longer. During these dead times the wafer being illuminated is either stepped (i.e., moved a few millimeters) or a treated wafer is replaced with a to-be-treated wafer or a new cassette of wafers is moved into place. In addition to controlling energy sigma, it is in many situations even more important to control the total dose during each pulse to as close as possible to a target value.
If the laser is operated in a burst mode at constant voltage, the result is a wide swing in pulse energy during the first 40 milliseconds of the burst. Prior art energy feedback control circuits only partially reduce the energy variations. It is known that the swings are relatively repetitive from burst to burst. Therefore, attempts have been made to program the voltage control with time dependent algorithms in order to reduce the pulse energy variation during these first 40 pulses.
What is needed is better techniques for reducing the energy sigma during burst mode operation.